What is the percentage of oxygen found in the atmosphere? Name two compounds of oxygen found in nature. Name any three processes in which oxygen is used up in the atmosphere.

(N/A) The percentage of oxygen in the atmosphere is approximately $21 \%$.
Two compounds of oxygen found in nature are water $(H_2O)$ and carbon dioxide $(CO_2)$.
Three processes in which oxygen is used up in the atmosphere are:
$(i)$ Respiration: Living organisms consume oxygen to break down glucose and release energy.
$(ii)$ Combustion: Burning of fuels requires oxygen to proceed.
$(iii)$ Formation of oxides of nitrogen: During lightning,nitrogen reacts with oxygen to form nitrogen oxides.